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Evolution and Population Genetics
Evolution Occurs in Populations
Species
There is not a good definition of species; perhaps the concept of species is artificial but it is useful
because it allows people to classify organisms.
Most biologists would agree that members of a sexually-reproducing species are able to interbreed
and have a shared gene pool.
Different species do not exchange genes with each other; they do not interbreed.
This definition of species is based on sexual reproduction and therefore does not work with
prokaryotes or other asexual species.
Population
A population is an interbreeding group of organisms (the same species) that occupies a particular
area.
The size of the area is somewhat arbitrary. There could be a population of fish in an aquarium and a
population of fish in a lake.
Gene Frequency and Evolution
Gene frequency refers to the proportion of alleles that are of a particular type. For example, if 60%
of the alleles in a population are "a" and 40% are "A", then the gene frequency of "a" is 0.6 and the
gene frequency of "A" is 0.4.
On a small scale, evolution involves changes in gene frequencies.
Population Model
A population is a group of interbreeding organisms that occupies a particular area.
Initial Population
Circles are used to represent genes in this diagram of a population. Individuals are diploid, so two
circles are used to represent an individual.
Gene Frequencies in the Model Population
In the population above, 33% of the genes for eye color in a population are "A" and 67% are
"a". The frequency of "A" is therefore 0.33 and the frequency of "a" is 0.67.
Gametes
During meiosis, "AA" individuals will produce all "A" gametes. Similarly, 1/2 of the gametes
produced by an "Aa" individuals will be "A" and the other half will be "a"; "aa" individuals will
produce all "a" gametes.
Individual
AA
Aa
aa
Gametes
all A
1/2 A, 1/2 a
all a
The proportion of A and a in the gametes will be the same as in the population. In the example
population we have been using, suppose that each individual produces four gametes.
In reality, males produce many millions of gametes and females produce relatively few. This is not
a concern for our model because in either case, the gene frequency of the gametes will be the same
as that of the population that produced them.
The gene frequency of "A" and "a" in the gamete pool will remain 0.33 and 0.67.
Gene frequency: The next generation
Because the gene frequency in the gamete pool did not change, the gene frequency in the population
the next generation remains the same.
The Hardy-Weinberg law states that under certain conditions (discussed below), the gene frequency
of a population does not change from generation to generation.
Should There Be Fewer Recessive Alleles?
The population model described above predicts that gene frequencies will not change from one
generation to the next even if there are more recessive alleles.
There is sometimes a misconception among students beginning to study genetics that dominant
traits are more common than recessive traits. It isn't true. For example, blood type O is recessive
and is the most common type of blood. Huntington's (a disease of the nervous system) is caused by
a dominant gene and the normal gene is recessive. Fortunately, most people are recessive; the
dominant is uncommon.
The misconception comes from the observation that in a cross of Aa X Aa, 3/4 of the offspring will
show the dominant characteristic. However, the 3:1 ratio comes only if the parents are both Aa. If
there are many recessive genes in a population, then most matings are likely to be aa X aa and most
offspring will be aa.
Forces that Change Gene Frequencies
Migration can change the gene frequency of a population if the migrants have a different gene
frequency than that of the population they are leaving or entering.
The founder effect occurs when the gene frequency of a newly established population is somewhat
different from the parental population. This may be due to the small sample of founding individuals.
The sample-size phenomenon can be illustrated by flipping a coin. The expected number of "heads"
from flipping a coin is 50% but if a coin is flipped only 4 times, you may get all "heads" or all
"tails". If the coin is flipped 1000 times, the actual number of "heads" and "tails" will probably not
deviate much from 50%. Thus, the larger the sample size of emigrants, the more likely it is to reflect
the population from which it is leaving.
Below: The population on the right was formed from a few individuals emigrating from the
population on the left.
During a bottleneck, a large population undergoes a decrease in size so that relatively few
individuals remain. Because there are few individuals, the gene frequency is more likely to drift.
Below: The gene frequency of the initial population (left) changes because many of the individuals
have died. The population on the right is the same population after the bottleneck has occurred.
Genetic drift refers to random fluctuations in the gene frequency of a population. This is more
likely to occur in a small population. As with bottlenecks and the founder effect, it is a sample-size
phenomenon. The smaller the population, the more likely that gene frequencies are likely to
fluctuate from generation to generation.
Mutation changes gene frequencies when genes of one type ("A" for example) mutate to another
type ("a" for example).
Natural selection changes gene frequencies when genes or gene combinations are more likely to
result in greater reproductive success of the individual that possesses them.
Conditions Necessary for Hardy-Weinberg Equilibrium
Notice that the gene frequency the next generation is the same as that of the initial population. The
Hardy-Weinberg principle states that if the following conditions are met, the gene frequency of a
population will not change from generation to generation:
No migration
Large population size
No mutation
Random mating
No selection
Natural Selection
Natural selection is a mechanism that produces changes in the gene frequency from one generation
to the next. As a result, organisms become better adapted to their environment. It is important to
keep in mind as you read below that natural selection does not act on individuals; it acts on
populations. Individual organisms cannot become better-adapted to their environment.
Natural selection occurs because
1.
2.
3.
4.
Individuals within a population vary; they are not all identical.
Some variants are “better” than others.
The traits that vary are heritable.
The “better” individuals will have more success reproducing; they will have more offspring.
In successive generations, more offspring will have the better trait.
These items are discussed below.
Variation
Sexual reproduction promotes genetic variation.
For many traits that occur in a population, individuals are often
not all identical. For example, if running speed were measured,
some individuals would likely be able to run faster than others but
most individuals would probably be intermediate.
If number of individuals is plotted against the trait in question
(running speed for example), a graph like the one shown is often
produced.
We would get a similar bell-shaped curve if we plotted height,
weight, performance on exams, or almost any other characteristic.
Some Variants are Better
Some individuals are bound to be better than others. Perhaps their body structure allows them to
escape predators better or to find food faster or to better provide for their young. For example,
suppose that the faster-running animals diagrammed below are better able to escape predators than
the slower ones. You would expect that more of the faster ones would survive and reproduce than
the slower ones.
The slower rabbits will not reproduce as much because predators kill them more than they kill the
faster rabbits.
Traits Are Heritable
Those individuals that survive better or reproduce more will pass their superior genes to the next
generation. Individuals that do not survive well or that reproduce less as a result of "poorer genes"
will not pass those genes to the next generation in high numbers. As a result, the population will
change from one generation to the next. The frequency of individuals with better genes will increase.
This process is called natural selection.
Natural Selection Produces Evolutionary Change
If the conditions discussed above are met, the genetic composition of the population will change
from one generation to the next. This process is called natural selection.
The word "evolution" refers to a change in the genetic composition of a population. Natural
selection produces evolutionary change because it changes the genetic composition of populations.
A variety of other mechanisms can also produce evolutionary change. For example, suppose that
65% of the eye-color genes in a population were for individuals with blue eyes and 35% of the
genes were for brown eyes. If most of the immigrants entering the population carried the blue gene,
the overall composition might change from 65% blue to 70% blue.
Natural selection acts on populations; a single individual cannot evolve. Natural selection does not
act on an individual to make it better adapted to its environment.
Example of Natural Selection: Industrial Melanism
Kettlewell studied the peppered moth (Biston betularia) from insect collections in England. He
observed that in polluted areas, most of the peppered moths were the dark form. In clean areas, most
were the pale form.
During the early 1800's, the dark form comprised less than 2% of the population and the pale form
made up more than 98%. During the 1800’s the dark form increased in frequency in urban areas.
Kettlewell suggested that dark moths survived better in polluted areas because they were more
difficult for avian (bird) predators to see on the darkened tree trunks. Similarly, he suggested that
light-colored moths were more difficult to see in unpolluted areas because the tree trunks were
light-colored.
To test this, he released moths of each type (light and dark) in both polluted and unpolluted areas.
In the unpolluted area, he recaptured 13.7% of the light moths and 4.7% of the dark moths. In the
polluted area, he recaptured 13% of the light and 27.5% of the dark moths.
Sexual Reproduction and Evolutionary Change
Variation
Individuals with in a population usually are not all identical and much of this variation is due to
genetic differences among individuals.
Sexual reproduction acts to increase variation in populations by shuffling genes. Offspring have
some genes from each of two different parents and therefore are not identical clones of their parents.
The increased variation due to sexual reproduction allows natural selection (and thus evolution) to
produce changes in populations as described above.
Ultimately, all variation in a population comes from changes in the DNA. These changes are called
mutations.
Recombination during sexual reproduction promotes variation. Sperm and eggs (gametes) are
produced by a type of cell division called meiosis. During meiosis, crossing-over and independent
assortment act to shuffle the genes before gametes are produced.
Fluctuating environments
Evolutionary change due to natural selection would not be necessary if the environment never
changed and the organisms within the environment were optimally adapted to the environment. For
example, imagine a plant that is adapted to an environment that has an average annual rainfall of
100 cm. If the climate were to change so that the amount of rainfall decreased, individuals that
could tolerate less rain would survive and reproduce better, thus establishing their drought-tolerant
genes in subsequent generations. If there was no variation in the plant population, there would not
be any drought-tolerant individuals and the species would likely go extinct in areas of decreased
rainfall.
Sexual reproduction therefore, enables species to survive in fluctuating or changing
environments because it promotes variation, which in turn allows natural selection.
Speciation and Taxonomy
Speciation
Speciation refers to the formation of species. It occurs when there is reproductive isolation between
populations.
Allopatric refers to populations that are geographically isolated (they do not overlap). Sympatric
refers to populations whose ranges overlap. The diagram below is a map of the distribution of two
populations. Each population occupies a circular area. The two populations are allopatric in part of
their range but are sympatric (overlap) in part.
Allopatric Speciation
Allopatric speciation occurs when species are formed as a result of populations being isolated from
each other so that they do not interbreed.
Allopatric speciation is the most common type of speciation.
Hypothetical Example of Allopatric Speciation
The following hypothetical example illustrates how isolating two populations can lead to the
formation of two species.
Suppose...
Suppose that there are two populations of rabbits, each on an island in the ocean. One island has
predators that eat rabbits and the other does not. How might natural selection change the rabbit
populations after several thousand years?
Suppose that the rabbit food is different on one island than it is on the other. How might natural
selection change the rabit populations with regard to their food requirements after several thousand
years?
Suppose that the climate of one island is hot but the climate of the other is cold. How might the
rabbits become adapted to the different climates?
Eventually, the two populations will become so different, that they will be different species.
Reproductive Isolation
In the diagram below, the population is divided into two isolated populations by the uplift of a
mountain range between the two. After separation, evolution in one population will be independent
of evolution in the other because they are not exchanging genes.
Example: Finches in the Galapagos Islands
All of the native bird species living in the Galapagos may be descended from a single species that
arrived there from South America. The species now found there arose by allopatric speciation on
the many different islands.
Sympatric Speciation
Sympatric speciation refers to the formation of species from a population that is not divided
geographically. In order for two (or more) species to emerge from this population, individuals must
become reproductively isolated.
Speed of Evolutionary Change
Two hypotheses have been proposed to explain the rate of evolutionary change.
Gradual Change
The gradualist explanation says that species change (evolve) slowly over time.
Example
Some characteristics of trilobites changed steadily over 3 million years.
Punctuated Equilibrium
The punctuated equilibrium explanation says that periods of slow evolutionary changed are
interspersed (punctuated) with periods of rapid change.
This explains why there are often fewer than expected numbers of intermediate forms in the fossil
record. Intermediate forms exist only briefly during a period of rapid change.
Evolutionary change occurs rapidly during the early part of a species history because the species
may not be well adapted to its environment. For example, if the food of a bird species is such that a
long bill is needed to feed, you would expect evolution to produce long bills very rapidly.
The pace of evolutionary change slows down as species become adapted to their environments.
Examples
The organisms living in a forest in northern New York are well adapted to their environment
because they have lived there for thousands of years. If the climate were to start changing, the rate
of evolutionary change is expected to increase so that organisms can remain adapted to their
environment.
Evidence indicates that life first evolved in the ocean. When organisms first invaded the land,
evolutionary change was rapid; major changes were needed for success. As they became adapted to
terrestrial environments, the pace slowed down. Generally, you would expect the rate of evolution
to be rapid when organisms are expanding into new environments that they are not ideally adapted
to.
Taxonomy
Taxonomy refers to the classification of living organisms. Classification is based on evolutionary
relationships.
The diagram below shows hypothetical evolutionary relationships among a number of different
organisms. Each vertical line represents a different kind of organism. Ancestral individuals are
indicated by vertical lines near the top of the diagram and closely related kinds are linked to these
by horizontal lines.
Individuals of the same kind are categorized as a species. In sexually-reproducing organisms, two
individuals of the same species are capable of interbreeding. Closely-related species are grouped
into a genus. Closely-related genera are grouped into a family, then order, class, phylum (or
division in plants), kingdom, and finally domain.
Many taxonomists categorize organisms into 3 domains: Bacteria, Archaea, and Eukarya. The
Bacteria and Archaea are procaryotes. The domain Eukarya is the eukaryotes. It is composed of
these kingdoms: Protists, Fungi, Plants, and Animals.